Microparticles are solid or semi-solid particles having a diameter of less than one millimeter, more preferably less than 100 microns, which can be formed of a variety of materials, including synthetic polymers, proteins, and polysaccharides.
Exemplary polymers used for the formation of microspheres include homopolymers and copolymers of lactic acid and glycolic acid (PLGA) as described in U.S. Pat. No. 5,213,812 to Ruiz, U.S. Pat. No. 5,417,986 to Reid et al., U.S. Pat. No. 4,530,840 to Tice et al., U.S. Pat. No. 4,897,268 to Tice et al., U.S. Pat. No. 5,075,109 to Tice et al., U.S. Pat. No. 5,102,872 to Singh et al., U.S. Pat. No. 5,384,133 to Boyes et al., U.S. Pat. No. 5,360,610 to Tice et al., and European Patent Application Publication Number 248,531 to Southern Research Institute; block copolymers such as tetronic 908 and poloxamer 407 as described in U.S. Pat. No. 4,904,479 to Illum; and polyphosphazenes as described in U.S. Pat. No. 5,149,543 to Cohen et al. Microparticles produced using polymers such as these exhibit a poor loading efficiency and are often only able to incorporate a small percentage (typically less than 10%) of the drug of interest into the polymer structure.
These microparticles have a wide particle size distribution, often lack uniformity, and may not exhibit desired release kinetics. Furthermore, the polymers used are dissolved in organic solvents in order to foil these microparticles. They must therefore be produced in special facilities designed to handle organic solvents. These organic solvents could adversely affect the drug contained in the microparticles. Residual organic solvents could be toxic when administered to humans or animals.
In addition, the available microparticles are rarely of a size sufficiently small to be useful for administration by inhalation. For example, microparticles prepared using polylactic glycolic acid (PLGA) are large and have a tendency to aggregate. A size selection step, resulting in product loss and cost increase, is necessary.
Microparticles prepared using lipids to encapsulate target drugs are known. For example, lipids arranged in bilayer membranes surrounding multiple aqueous compartments to form particles may be used to encapsulate water soluble drugs for subsequent delivery, as described in U.S. Pat. No. 5,422,120 to Sinil Kim. These particles are generally greater than 10 microns in size and are designed for intra-articular, intrathecal, subcutaneous and epidural administration. Alternatively, liposomes have been used for intravenous delivery of small molecules. Liposome technology has been hindered by problems including purity of lipid components, possible toxicity, vesicle heterogeneity and stability, excessive uptake and manufacturing or shelf-life difficulties.
An objective for the medical community is the delivery of nucleic acids to the cells of a subject, including but not limited to an animal or a mammal, for treatment. For example, nucleic acids can be delivered to cells in culture (in vitro) relatively efficiently, but nucleases result in a high rate of nucleic acid degradation when nucleic acids are delivered to animals (in vivo).
In addition to protecting nucleic acid from nuclease digestion, a desirable nucleic acid delivery vehicle would exhibit low toxicity, be efficiently taken up by cells and have a well-defined, readily manufactured formulation. As shown in clinical trials, viral vectors for nucleic acid delivery can result in a severely adverse, even fatal, immune response in vivo. In addition, this method has the potential to have mutagenic effects in vivo. Delivery by enclosing nucleic acid in lipid complexes (such as liposomes or cationic lipid complexes) has been generally ineffective in vivo and can have toxic effects. Complexes of nucleic acids with various polymers or with peptides have shown inconsistent results and the toxicity of these formulations has not yet been resolved. Nucleic acids have also been encapsulated in polymer matrices for delivery but in these cases the particles have a wide size range and the effectiveness for therapeutic applications has not been demonstrated.
Therefore, there is a need for addressing nucleic acid delivery issues, and providing effective nucleic acid formulations. Also, there is an ongoing need for development of microparticles and to new methods for making microparticles. Microparticles and their preparation have been described in U.S. Pat. No. 6,458,387 to Scott et al., U.S. Pat. No. 6,268,053, U.S. Pat. No. 6,090,925, U.S. Pat. No. 5,981,719 and No. 5,599,719 to Woiszwillo et al., and U.S. Pat. No. 5,578,709 to Woiszwillo, as well as U.S. Publication No. 20050142206 and U.S. Publication No. 20060018971. Each of the foregoing references and all other references identified therein and herein are incorporated herein by reference. It is noted, however, that these microparticles previously described typically were prepared using a polymeric cation such as, for example, poly-L-lysine or poly L-ornithine. While the use of such polymeric cations produces excellent results with microparticles having nucleic acid loading of 20 weight percent to 100 weight percent, and having an average particle size of not greater than about 50 microns, typically, the polymeric cations render these microparticles relatively insoluble in water. Therefore, these microparticles of polymeric cations and nucleic acids are not suitable for releasing nucleic acids at target locations. While such microparticles may be taken up wholly by certain target cells and/or other cells (e.g., macrophages) through endocytosis, these microparticles do not dissolve at a target site that has an aqueous environment and hence the nucleic acids in these microparticles cannot interact freely with such target cells.
As such, there remains a need for microparticle preparations that readily dissolve at target locations that are in a moist or aqueous environment such as, for example, the lungs, nasal membranes, mouth, throat, stomach, intestines, vagina, any parts of the respiratory system, open wounds (e.g., lesions, lacerations, surgical wounds, burn wounds), any mucosal membranes, any epithelial cells, any vasculature, and the like to release nucleic acids that can freely interact with the target cells.